Quick smoke + fire not rendering properly in 2.81a
In the end, it was possible to "jog" Blender into rendering a frame of a burning basic torus using the default settings for Quick Smoke + Fire. I changed CUDA settings to CPU only and replaced the Output shader with a fresh default one. The fire + smoke then F12 rendered, if slowly.

When I then turning GPU Compute back on (and all GPUs + CPU on as well in CUDA settings in Preferences), the fire + smoke still rendered, and a lot faster. Have the Blender devs added GPU support to Cycles with 2.80? If so, thank you! I just find it strange that the Material Output shader needed replacing to get the physics effect to run.